1. Field of the Invention
The present invention relates to a transmission method using the Orthogonal Frequency Division Multiplexing (OFDM) method.
2. Description of the Related Art
In recent years, a transmission method called the Orthogonal Frequency Division Multiplexing (OFDM) method has been proposed as a method for transmitting digital signals. In the OFDM method, pieces of data are assigned to a plurality of carrier waves that are orthogonal to one another on the frequency axis, and the modulation and demodulation are performed on the basis of the IFFT and FFT. The OFDM method can realize high efficiency in frequency utilization, and hence the application of the OFDM method to digital terrestrial broadcasting is widely discussed. Also, the OFDM method is employed as one of the standards of the ISDB-T (Integrated Services Digital Broadcasting-Terrestrial) for digital terrestrial broadcasting in Japan.
In a conventional digital modulation method that uses a single carrier wave, the higher the transmission rate, the shorter the symbol period becomes, and accordingly the demodulation of signals has been difficult in multi-path situations (situations in which a wave transmitted from a base station is reflected by obstacles such as buildings or the like, and comes to be received by a reception terminal [such as a mobile phone or a TV receiver] via a plurality of paths). In order to cope with this situation, in the OFDM method, the multi-carrier transmission method is used (information is divided and transmitted on a plurality of carrier waves), and thereby a greater symbol length on one carrier wave is obtained in order to cope with multi-path situations involving great delays. Also, because pieces of data are assigned to a plurality of carrier waves, different modulation method scan be used over such carrier waves.
Also, in the OFDM method, a signal at the end portion of the symbol is added before the symbol, and thereby tolerance against multi-path situations is enhanced. The signal thus added is called a guard interval.
FIG. 1 shows the effect of the guard intervals in the multi-path situations. FIG. 1A shows the case when the guard interval is not used, and FIG. 1B shows the case when the guard intervals are used.
In FIG. 1A, a guard interval is not used, and thus when the FFT process is performed on the symbol “n” in a situation with a delayed wave (a signal wave that arrives delayed after being reflected by objects) in addition to the principal wave (a signal wave that arrives directly), a part of the data of the symbol “n−1” of the delayed wave is involved. This causes interference between the successive symbols, and a deterioration is also caused. In contrast, in the case of FIG. 1B where the guard intervals are used, a part of the data of the symbol “n−1” is not involved when the FFT process is performed on the symbol “n”. Thus, the demodulation can be performed without causing the interference.
In the ISDB-T for digital terrestrial broadcasting in Japan, for example, one of three lengths (¼, ⅛, and 1/16 of the symbol length) is assigned to the guard intervals, and these guard intervals are automatically identified.
Also, two standards are employed for the spacing between the carrier waves in the OFDM, i.e., mode 2 that specifies 0.504 ms as the length of one symbol, and mode 3 that specifies 1.008 Bms as the length of one symbol. Also, the guard intervals are added to them. The transmission modes are also automatically identified.
FIG. 2 shows a conventional transmission mode/guard length determination circuit 91. The transmission mode/guard length determination circuit 91 includes a one-symbol delay circuit 92, a correlation calculation circuit 93, a moving average circuit 94, a maximum-value detection circuit 95, a maximum-value integral circuit 96, and a maximum-value comparison circuit 97.
The one-symbol delay circuit 92 delays real data in the principal wave by one symbol (however, the guard intervals are not included).
The correlation calculation circuit 93 correlates the real data in the principal wave and the data delayed by one symbol by the one-symbol delay circuit 92 (not including the guard intervals). The portions of the guard intervals have the same data, and accordingly a high correlation is obtained between them (guard correlation). In other words, when attention is paid to the symbol “n” in FIG. 3, the end portion of the symbol “n” of “(1) principal wave” is correlated with the guard interval Gin of the symbol “n” of (2) the one-symbol delay at the same timing, and it is possible to detect the peak values as shown in “(3) guard correlation”. The moving average circuit 94 performs the moving average calculation on the correlation value of the correlation calculation circuit 93 for the guard length. As a result of this, as shown in “(4) moving average” in FIG. 3, peaks are caused at boundary of the guard intervals and the real data.
The maximum-value detection circuit 95 detects a peak value that is the maximum value of signals output from the moving average circuit 94. The maximum-value integral circuit 96 integrates, for several symbols, the peak value from the maximum-value detection circuit 95, and outputs a peak integral value. The maximum-value comparison circuit 97 compares a predetermined threshold value with the peak integral value, and determines a combination of a transmission mode and a guard length on the basis of the comparison result.
Usually, a high peak is not caused unless the magnitude of the peaks are integrated for several symbols and a moving average of the guard correlation is obtained on the basis of the right combination of the transmission mode and the guard length. Accordingly, the results of the combinations of all the transmission modes and guard lengths in the standard are compared among one another, and the combination of the highest transmission mode and guard length is determined to be the right value.
In Patent Documents 1 and 2, a demodulation of OFDM signals is proposed in which transmission modes such as effective symbol lengths and guard intervals are recognized by using the above conventional method.
According to Patent Document 3, a symbol synchronization unit 16 comprises a first detector 401, a second detector 402, and a third detector 403 detecting a reception OFDM signal x(n), a logic AND element 41 performing the AND operation on detected signals v1 (n), v2 (n), and v3 (n) output from the detectors 401 through 403 in order to output a logical signal AV, and a determination circuit 42 generating and outputting a synchronization timing signal T0 by using the logical signal AV. Also, the symbol synchronization unit 16 detects a mode for the reception OFDM signal x(n) from among three modes (modes 1, 2, and 3), and the initial points, the guard interval lengths, and the effective symbol lengths in the respective symbols are automatically detected.
However, when mobile phones or mobile bodies receive data of digital terrestrial broadcasting, fading is frequently caused in which the reception level of radio waves changes due to the movement of the receivers or to elapses of time. In a situation having fading that is too intensive, the electric power itself weakens for a particular time period, and accordingly in this period, the transmission mode and guard length cannot be determined correctly.
Also, when the electric power accidentally weakens for the guard correlation calculation for a target transmission mode and guard length while the transmission mode and guard length are being detected by using a conventional method under the fading situation, another combination comes to have a higher correlation value, and the combination is wrongly selected. When this happens, the wrong transmission mode and guard length are used for the operation and nothing can be received until the fact that the currently used combination is wrong is recognized and the right transmission mode and guard length are detected.
As described above, in fading situations, it takes a longer time to select the right transmission mode and guard length and demodulation processes cannot be performed at all until the right transmission mode and guard length are selected, which is problematic.
Patent Document 1
Japanese Patent Application Publication No. 2003-46472
Patent Document 2
Japanese Patent Application Publication No. 10-327122
Patent Document 3
Japanese Patent Application Publication No. 2003-264528